The present invention relates in general to providing circuit protection from excessive voltage in applications having a high voltage swing. More particularly, the invention relates to providing circuitry capable of providing a voltage swing at the output in excess of the voltage swing tolerated by the individual circuit components. More specifically, the invention enhances cascode topology such that relatively low voltage transistors may be used in a circuit designed for higher voltage applications.
It is well known in the semiconductor arts to use transistors for controlling circuit output. Frequently circuits requiring high voltage supplies can tax the limits of a circuit output transistor or other circuit components. In general, to ensure proper circuit operation larger capacity circuit components are used in high voltage applications. For example, a typical milliactuator driver may require a voltage swing of approximately xe2x88x9225 Volts to +25 Volts. The typical solution in such applications is to use transistors capable of withstanding a drain-to-source voltage (VDS) of approximately 50 Volts. In this way the circuit is capable of withstanding the anticipated voltage swings.
Problems arise, however with this traditional solution. Often manufacturing processes affect the ability to select components. Such a problem may arise for example, where a circuit previously designed and manufactured with processes using transistors capable of withstanding 50 Volts is to be implemented using manufacturing processes superior in other respects, but limited to producing transistors capable of withstanding only 30 Volts. It would be useful and advantageous in the arts to provide apparatus, systems and methods for protecting relatively low voltage circuit components from large swings in voltage while permitting large voltage swings at the output of a circuit.
In general, the present invention provides novel apparatus, methods and systems for protecting circuit components in a circuit having large voltage swings. More specifically, the invention protects individual transistors from voltage swings higher than their maximum capacity in circuits capable of sustaining voltage swings higher than the maximum capacity of the individual transistors by providing a protective subcircuit adjustable in response to the output voltage.
According to one aspect of the invention, a method for providing excess voltage protection in a circuit having a voltage swing greater than the voltage capacity of the circuit output transistor includes the step of maintaining the gate-to-source voltage of a protection transistor in a range from approximately zero to the maximum allowable gate-to-source voltage. A further step includes maintaining the drain-to-source voltage of the protection transistor in a range from approximately zero Volts to the maximum allowable drain-to-source voltage. In a further step, the drain-to-source voltage of the circuit output transistor is maintained below its maximum.
According to a further aspect of the invention, a step of adjusting the protection transistor gate-to-source voltage in response to the circuit output voltage is provided.
According to an additional aspect of the invention, a system for providing circuit voltage swing capacity in excess of the maximum voltage swing capacity of individual circuit components is provided. The system includes a circuit having an output transistor for controlling circuit output. A protection transistor is provided for protecting the output transistor from excessive drain-to-source voltage. A follower section is provided for protecting the protection transistor from excessive gate-to-source voltage and for preventing excessive drain-to-source voltage at the protection transistor. A biasing section is used for providing bias current to the follower section responsive to circuit output voltage.
In a preferred embodiment of the invention, a protection transistor is implemented using PMOS, and a follower section transistor is implemented using NMOS.
According to yet another aspect of the invention, apparatus for protecting a circuit from excessive voltage is provided. A protection section is operably coupled to an output transistor of the circuit and an adjustable follower section. The adjustable follower section includes a follower transistor coupled to the protection transistor for adjusting the bias of the protection transistor in response to changes in circuit output voltage. A biasing section is coupled to the follower section for supplying bias current to the follower transistor.
According to still another aspect of the invention, a preferred embodiment implements the apparatus of the invention using PMOS transistors for output and protection transistors and an NMOS follower transistor.
The invention provides many technical advantages including but not limited to providing a circuit output capable of swinging within a broader voltage range that the maximum drain-to-source voltage of the circuit output transistor. Further advantages are realized in terms of reduced complexity and circuit area. The invention provides additional advantages by introducing a variable protection section biasing scheme, which increases the utility of relatively low voltage components for high voltage applications and increases overall flexibility in terms of component selection in circuit design.